The invention relates to a device for producing print images of varying lengths in offset printing, the device comprising format sections with different cylinder diameters which can be exchanged for changing the print image length.
A device for producing print images of varying lengths in offset printing is known from German patent document DE 199 55 084 A. This device uses printing sleeves with a changeable diameter which are fitted onto cylinder cores and/or base cylinders for a full-rotation offset printing machine. The cylinder cores are positioned with both ends inside a frame, wherein one side of the frame is provided with a drive unit for the cylinder cores. The other side of the frame is used for the removal of the printing sleeves. The cylinder cores must take the form of air shafts, so that the printing sleeves can be fitted over the cylinder cores. The printing sleeves, which form the format sections, must be provided with a compressible layer on the inside surfaces, allowing the sleeves to be expanded somewhat and fitted over the air shaft. The disadvantage of these elastic and/or compressible layers, however, is that the high line forces generated in offset printing push the printing sleeves from the concentric position. This leads to changes in the adjustments and thus considerable printing problems. Air shafts and/or printing sleeves of this type have long been known in flexographic printing. However, since considerably lower pressure forces occur in the printing gap during flexographic printing, the aforementioned problems do not arise.
A further difficulty with this device is the torque transmission. The air shaft of known print units in offset printing has a specific diameter that is based on the smallest print length to be produced. For example, if this print length must start at approximately 350 mm (13.8″ circumference), then the air shaft should have a maximum diameter of 105.158 mm. The same air shaft is also used for a print length of approximately 609 mm (24″, D=194.04 mm). In that case, the air shaft diameter is not favorable and can lead to considerable problems in the printing sleeve composition and the torque transmission.
European patent document EP 1 101 611 A also relates to a device for producing print images of varying lengths in offset printing, wherein this device also has replaceable printing sleeves. These printing sleeves are positioned on pivoting arms. For the printing sleeve replacement, openings are worked into one of the two bearing plates through which the printing sleeves can be exchanged. However, the aforementioned problems also exist with this method and/or device.
German patent document DE 35 43 704 A relates to a device for imprinting a web. The sleeves in this case are also attached so as to be replaceable on a rotating body. A bearing plate can be folded down on the operating side for replacing the sleeves. However, producing such a fold-down bearing plate for an offset press is costly.
German patent document DE 19 53 4651 relates to a printing unit for indirect printing. For this, respectively one form cylinder and one transfer cylinder can be fitted for a variable circumference with so-called sleeves having variable outer diameters.